1. Field of the Invention
The invention relates to optical transmission systems, and, in particular, to techniques for responding to failures in such systems.
2. Description of the Related Art
Broadband Raman optical amplifiers are often used in long-reach, wavelength-division multiplexing (WDM) transmission systems. When transparent networking elements such as optical add-drop multiplexers (OADMs) and optical cross-connects (OXCs) are used in such systems, the amplifier power transients resulting from network failures can result in significant power excursions on WDM channels that survive the network failure. These excursions are caused by gain saturation and pump depletion in the optical amplifiers. The desired amplifier gain settings and pump power levels for the surviving channels will change depending on the number of WDM channels being amplified and their configuration within the WDM spectrum. Transient spiking and residual gain errors may occur unless the optical amplifiers are controlled to respond sufficiently to such events.
For broadband Raman amplifiers, multiple pumps supply power at different wavelengths, where WDM channels at different wavelengths can receive different amplification depending on the various pump power settings. Setting the correct pump powers in order to achieve a target gain for each WDM channel is a complex operation that is usually accomplished by measuring the power levels of one or more channels and adjusting one or more pump powers so that the channel powers approach the target levels. These measurements and pump adjustments are often performed many times in order to get a set of channels to match their target power levels. During normal system operation, these measurements and adjustments happen at a relatively slow rate (e.g., typically seconds to minutes).
The control response of an amplifier to a transient event should be fast enough to sufficiently suppress power excursions in surviving WDM channels. The conventional Raman amplifier adjustment procedure mentioned above will typically be too slow to respond to many common network failures such as fiber breaks.